For the numbered items:
(1) and (2) At takeoff there is (typically, but there are birds) nothing solid that's airborne in front of the airplane, whereas thrust reversers can put small but solid airborne material in front of the engines, especially if they're still being used with high power as the airplane slows.
(3) If you're saying that the air is only directed sideways rather that forward, that would cancel the forward thrust from that air, but it wouldn't get you much braking action.
Some other points:
- The higher the airplane's speed, the more braking action you get from the reversers, so if you're going to use them, get into them as soon as possible.
- If it's late night or early morning, and the airport has a noise problem, consider going into reverse but leaving the engines at idle. You'll still get significant braking proportional to the airplane's speed.
- If you've got a long runway, consider using just idle or less than high-power reverse. In other words, use what you need/want, but no more.
- Make sure you start out of a high power reverse well before slowing to taxi speed. There are speed guidelines for this, but I forget what they are. Actually, it's best in my opinion to avoid high-power reverse unless you really need it.
- Make sure you're completely out of reverse by the time you reach taxi speed. Again, I've forgotten the guideline.
- Consider where you are. If you're in, say, Germany, you can expect the runways to be clean, in Spain maybe not as clean, in Zimbabwe definitely not as clean. You get the idea.
- Be really careful about using reverse on an icy runway. If your nose is not pointed straight down the runway, the reverse thrust will be working to put you off the side of the runway. An acquaintance of mine put a 747 off the runway at KJFK that way (totaled the airplane, a few people hurt, no fatalities).
- Consider the characters of the engines on the airplane. Perhaps this is no longer a consideration, but back in the 1990s, P&W engines on the 747 were relatively tolerant of compressor stalls brought on by using too much reverse power at too slow a speed. GE engines, however, were not so tolerant.
The second 747 carrier I worked for used to have maintenance done at Santa Barbara, CA. When we would pick up the airplane, they would close the relatively narrow runway after our takeoff and sweep it. Not a reverse thrust situation, but it does illustrate the FOD problem.